Saponite

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Calcium sodium magnesium iron aluminium silicate hydroxide
Chemical Formula: (Ca,Na)0.3(Mg,Fe)3(Si,Al)4O10(OH)2•4H20
Method(s) of Verification: Llŷn - XRD (Roberts, 1981); Llanelwedd Quarry - EMPA (Garvie & Metcalfe, 1997; Metcalfe, 1990).

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal
  • Metamorphic: low-grade
Introduction: saponite belongs to the smectite group of clay minerals. It has a greasy lustre and is found in a variety of colours from white or cream through to blue, green or red-tinged. Saponite is found as a low-temeprature alteration mineral in hydrothermal mineralized veins; infilling vesicules in basalt; in fissures cutting calc-silicates, iron-rich skarns, amphibolites, and in serpentinite (Anthony et al., 1995). The presence of saponite is generally only confirmed by X-ray diffraction or microprobe analysis.
Occurrence in Wales: as with many other low-grade metamorphic and hydrothermally produced secondary silicate minerals there are just a few reported occurrences in the literature. It is however likely that the distribution of such minerals, including saponite, is far wider than is currently documented.

Key Localities:

  • Llanelwedd Quarry, Builth Wells, Powys: saponite has been described from quartz-calcite veins cutting altered basic lavas at Llanelwedd Quarry, near Builth Wells as part of the hydrothermal assemblage (Garvie & Metcalfe, 1997, Metcalfe, 1990).
  • Llŷn, Gwynedd: Roberts (1981) reports saponite-bearing metadolerite sills from near Pwllheli and Dinas, on Llŷn, North Wales, forming intergranular patches or pseudomorphing olivine. It occurs with an assemblage containing actinolite ± albite ± ankerite ± calcite ± chlorite ± prehnite ± quartz ± titanite. A specimen of saponite from Gimlet Quarry is in the collections of the Natural History Museum ( BM 1909,462).

There are no key localities for this specimen.

References:

  1. Anthony J., Bideaux R., Bladh K. & Nichols M., 1995. Handbook of Mineralogy, vol. 2 (parts 2/1 and 2/2), Silica, Silicates
  2. Garvie, L.A.J. & Metcalfe, R., 1997. A vein occurrence of co-existing talc, saponite, and corrensite, Builth Wells, Wales. Clay Minerals, 32, 223-240.
  3. Metcalfe, R., 1990. Fluid-rock interaction and metadomain formation during low-grade metamorphism in the Welsh marginal basin. Unpublished Ph.D. thesis, University of Bristol.
  4. Roberts, B., 1981. Low grade and very low grade regional metabasic Ordovician rocks of Llyn and Snowdonia, Gwynedd, North Wales. Geological Magazine, 118, 189-200.

There are no references for this specimen.